Honestly I don't have a problem with the idea of Lennier committing some kind of betrayal out of love for Delenn

That is a good point. He would do ANYTHING for her. But then what actually happens is he essentially betrays her in hopes of getting a chance with her. It IS in line with his weird bitter behavior throughout the fifth season, but it isn't in line with everything we learned about his character in previous seasons. I will definitely have more to say on that when I get to that particular episode in this rewatch (I've slowed down a bit, which tends to happen in the fifth season, but I've also just been a lot busier than before. Still going, though!)

I too would have liked to have seen some scenario like the one you suggest. It would have been a better fit. But alas.

I don't like how bitter Lennier has become, and how he seems convinced that if he trains hard enough he can be with Delenn. I know he's young and inexperienced, but does he really not understand that that's not how these things work? And what about his Ranger trainer? Does it not give him some sort of inner peace?

The #1 problem to me seemed to be that he simply didn't get enough focus in season 5 to have a "turn" built up and executed well. This episode is arguably a good example of that as he's not really given prime focus even though his story links into the arc more than anyone elses.

I think playing off Morden could have helped to setup the kind of "turn" I mentioned quite well. Basically have a conversation with him picking apart Lenniers world view, pointing out that unlike say Delenn he's never really dealt with moral grey areas or had to live with guilt from his previous actions.